La pupila cambia con la luz.

Because pupila is a feminine noun in Spanish.

In this sentence, la pupila means the pupil (of the eye).

A useful warning: pupila can also mean female pupil/student in other contexts, but here the context is clearly about the eye because of cambia con la luz.

• la pupila = the pupil of the eye
• el pupilo = a male pupil/ward, in other contexts

So the article la matches the noun pupila.

Cambia is the third-person singular present indicative of cambiar.

So it means:

• it changes
• sometimes, depending on context, it is changing

Here, the subject is la pupila, which is singular, so cambia must also be singular:

• la pupila cambia = the pupil changes

Other forms for comparison:

• yo cambio = I change
• tú cambias = you change
• él/ella cambia = he/she/it changes
• las pupilas cambian = the pupils change

Spanish often uses the singular to talk about something in a general, universal sense.

So La pupila cambia con la luz can mean something like:

• The pupil changes with light
• The pupil changes according to the light

It is not necessarily talking about only one specific eye. It can be a general statement about how the pupil behaves.

English can do this too in some contexts, but Spanish uses this kind of generic singular quite naturally.

Here, con means something like with, in response to, or depending on.

So cambia con la luz means the pupil changes with the light, meaning as the light changes or according to the amount of light.

This is natural Spanish. It does not mean the pupil is physically together with the light. It describes a relationship:

• more light -> the pupil changes
• less light -> the pupil changes

Other possible Spanish phrases with a similar idea could be:

• según la luz = according to the light
• dependiendo de la luz = depending on the light

But con la luz is short and natural.

Yes. Luz is a feminine noun, so it takes la:

• la luz = the light

This is something you simply learn with the noun. The ending -z does not automatically tell you the gender, so the article helps.

Examples:

• la luz intensa = the intense light
• mucha luz = a lot of light

Yes, potentially.

The Spanish present tense often covers both:

• it changes
• it is changing

In this sentence, though, the most natural reading is the general one:

• The pupil changes with the light

That sounds like a fact or general truth, not something happening right now in front of you.

If Spanish wanted to emphasize an action happening right now, it could say:

• La pupila está cambiando con la luz.

But in a general statement, the simple present cambia is the normal choice.

Spanish often prefers the definite article with body parts, especially when the owner is obvious or unimportant.

So Spanish commonly says:

• la mano = the hand
• los ojos = the eyes
• la pupila = the pupil

rather than always using:

• su mano
• sus ojos
• su pupila

In this sentence, it is not important whose pupil it is. The sentence is just making a general statement about the body part, so la pupila is more natural.

No, here cambiar is being used as a normal non-reflexive verb.

• La pupila cambia = the pupil changes

This is enough, because the subject itself is changing.

Using se cambia would suggest a different structure and often a different meaning, such as something being changed, changing oneself, or a more special construction depending on context.

For a basic physical change like this, cambia is the natural form.

In Spain, cambia is pronounced approximately like:

• KAHM-byah

A few details:

• cam- sounds like kahm
• bia sounds like bya
• the b in Spanish is softer than English b
• the stress is on cam: CAM-bia

So:

• cambia -> CAM-bia

Yes, absolutely. This sentence sounds like a general fact, the kind of thing you might see in a textbook, lesson, or explanation about the body.

Spanish often uses the present tense this way to express:

• general truths
• scientific facts
• habitual behavior

So La pupila cambia con la luz is a very normal way to say that the pupil responds to light.

The structure is:

• La pupila = subject
• cambia = verb
• con la luz = prepositional phrase

So it breaks down as:

• The pupil
  • changes
    • with the light

This is a very straightforward Spanish sentence:

subject + verb + complement

That makes it a good example of basic Spanish word order.